I propose to investigate the regulation of glomerular blood flow and the glomerular filtration rate at the level of the single nephron. These studies will apply intravital microscopic methods for the analysis of the microcirculation of the glomerulus in a unique renal preparation, the ophidian kidney. The ophidian kidney possesses superficial nephrons which allow access to the entire afferent and efferent glomerular arterioles as well as the glomerular capillaries. Utilizing the unique advantages of this preparation a comprehensive study of the mechanisms regulating glomerular perfusion and filtration will be undertaken. The dual-slit method for the measurement of RBC velocity and blood flow will be applied with the use of epifluorescence microscopy and video analytical techniques. In addition, the pressures across the afferent and efferent arterioles and in the glomerular capillaries will be measured with a servo-nulling micropressure system. The simultaneous determination of the pressure gradients and the flow rate will allow the continuous and simultaneous estimation of the vascular resistance of the afferent and efferent arterioles. These techniques will be utilized to establish the site of action, the quantitative response and the time course of regulatory mechanisms of glomerular blood flow: myogenic, metabolic, tubulo-glomerular feedback and the renin-angiotensin system. These techniques, together with new proposed methods for the continuous measurement of the single nephron filtration rate, SNGFR, and a microprotein assay for the estimation of plasma colloid osmotic pressure from selected sites, will allow the evaluation of the relationship of SNGFR to blood flow and pressure. These measurements will allow the calculation of the ultrafiltration coefficient of the glomeruli which with the estimation of the capillary surface area will provide an estimate of the capillary hydraulic conductivity. These methods will allow the assessment of the control of SNGFR by the interactions of the glomerular mesangium and capillaries. This approach, newly applied in renal physiology, will provide greatly increased resolution for the analysis of the regulation of glomerular blood flow and the determination of the glomerular filtration rate. These studies may have larger implications in the considerations of various hypertensive states and the development of renal failure.